Thanks for the reply. Certain that the coke blocks dont form a lintel themselves, more wondering if there is any precedence of them indicating the presence of an iron strap as I've heard mention of elsewhere?
Embarking on making our fireplace larger - both wider and taller. Read a lot of post and feeling fairly comfortable about extending the opening from 550 to 1000mm with a prestressed concrete lintel. Have taken further plaster away to see if there is an upper lintel above the brick arch over...